A major challenge in the development of light-emitting elements is to improve light extraction efficiency. Light extraction efficiency denotes the proportion of light, which comes out of a light-emitting element (into, for example, air or transparent resin that surrounds the light-emitting element), in light generated within the light-emitting element. A light-emitting element may have an optical refractive index of approximately 2.2 to 3.8, air may have an optical refractive index of 1, and transparent resin may have an optical refractive index of approximately 1.5.
For example, when a light-emitting element has an optical refractive index of 3.4, a portion of light generated within the light-emitting element may come out of the light-emitting element into air at a critical angle of approximately 17 degrees and into transparent resin at a critical angle of approximately 26 degrees. That is, light, which is almost perpendicular to a surface of a light-emitting element from among light generated within the light-emitting element, can come out of the light-emitting element.
In this case, the light extraction efficiency of the light-emitting element is approximately 2.2% when a portion of light generated within the light-emitting element comes out of the light-emitting element into air, and the light extraction efficiency of the light-emitting element is approximately 4% when the portion of the light generated within the light-emitting element comes out of the light-emitting element into transparent resin. The other portion of the light is reflected by the surface of the light-emitting element and trapped in the light-emitting element.